The Flow Index of Clays and Its Relationship with Some Basic Geotechnical Properties

Abstract The flow index (FI), which is the slope of the water content versus log10 of the blows number plot for the Casagrande cup method, is the measure of the rate at which soil mass loses its shear strength with increased water content. FI was plotted against the plasticity index (PI) of 59 clays; each clay had a different mineralogy and origin, and together they accounted for a wide range of plasticity values. A linear relationship with R2 = 0.91 was observed. However, if FI is plotted against the liquid limit (LL) and the plastic limit (PL) separately, although a good correlation with LL is observed, FI poorly correlates with PL. FI was also correlated with the activity of soil, cation exchange capacity, specific surface area, and water uptake. Predicted equations from PI and activity were established on 13 independent samples confirming that a good correlation against measured values and estimated values (from FI) exists. From the literature review, FI was also used to indirectly estimate PL. Results show a trend, albeit the results are scattered (R2 = 0.66 for the Casagrande cup method and R2 = 0.76 for the cone penetrometer method), thus indicating that PL cannot be directly obtained from FI.